What The Heck Is A 'Green Roof?'

It looks like an open-air rooftop greenhouse. Or perhaps a manicured mini park in the sky.

Green roofs – these secret islands of bucolic beauty that are rarely seen and never heard on the ground – are quietly moving in, blanketing buildings across the country in muted greens and pinks. They're taking over the tops of structures big and small, from the narrow green terraces of the Empire State Building to the country's most massive green roof installation, which sits atop a sprawling Ford truck plant in Michigan.

The Environmental Protection Agency defines a green roof as any “vegetative layer grown on a rooftop.” And while pitching a layer of protective sod on top of buildings and homes is nothing new (the vikings did this, too), what's happening now is a bit more high tech.

The modern 'green roof' concept was introduced in Germany about 40 years ago where more than 10 percent of the country's roofs have gone green. Unlike early Norse techniques, modern green roofing relies more on savvy materials science than grass seed or sod. There is a bed of hearty vegetation on the tippy top of the green roof - but below the green shrubs sits a multi-layered lightweight wind and rain-friendly contraption.

The succulent leafy sedum plants on this green roof are born thirsty. When the rains come, water soaks in to the plants, trickling slowly down to the lightweight mats and fibers below, instead of running directly into gushing storm drains.

Just as humans sweat to stay cool, green roofs use a process called 'evapotranspiration' to keep their buildings cooler, by using heat from the air to evaporate the water in the rooftop plants, fibers and mesh. In the winter, buildings also stay warmer, as the green roof works like an insulating blanket, prolonging the life of the roof, and driving heating costs down.

Green roofs still don't come cheap. In Washington D.C., the city provides reimbursements that run from $10 to $15 per square foot to cover installation costs, making The District the nation's #1 green roof builder. But other big businesses around the country are investing more resources in the pricey green tech because it drives down heating and cooling costs over the long term. And municipalities like having green roofs in the mix, because they curb rainwater runoff in city storm drains and they are just one more tool to help keep the steamy urban 'heat island' effect in check.

So think of a green roof like a fancy, organic carpet for the birds: Its layers of fibers, topped off with water-absorbing herbs and shrubs make the perfect perch for a mid-day pause from cruising city skies. And while green roofs are not suited for human-sized walkers, they do make our walks around the city streets below... just a bit cooler and more comfortable.